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 Surface Armor for Erosion Protection
Disadvantages are: Preparation of the bank slope is usually required, either
for geotechnical stability or to provide a smooth surface for proper placement
of the armor. This may result in high cost, environmental damage, and
disturbance to adjacent structures. The extent of earthwork associated with
an armor revetment will be especially significant if the existing channel
alignment is to be modified either by excavation or by placing fill material in
the channel.
Effective subaqueous placement of armor material in deep water or when
current velocities are high is often difficult and costly.
Some armor materials may require special measures to mitigate undesirable
aesthetic and biological characteristics.
Design considerations are: Armor must have sufficient weight and/or strength
to remain in place when subjected to hydraulic forces and impact from objects
carried by the stream. It must also prevent significant loss of bank material
from beneath it due to turbulence of flow or movement of groundwater.
All armor protection requires careful consideration of the geotechnical
stability of the bank, and sometimes a granular or fabric underlayment is
required for proper interior drainage of the bank material, or to prevent loss
of fine grained material through the armor.
7.1 STONE ARMOR
The following paragraphs outline the general description, advantages, and
disadvantages for most stone armors used as a bank stabilization method:
Stone armor can be placed in four general configurations, the most common
being a "riprap blanket." Other forms, known as "trenchfill," "longitudinal
stone toe," and "windrow" (referred to in some regions as "falling apron"),
can be very useful in certain situations.
A stone armor usually consists of "graded" stone, which is a mixture of a wide
range of stone sizes; the largest sizes resist hydraulic forces, and the smaller
sizes add interlocking support and prevent loss of bank material through gaps
between the larger stones. Hand-placed stone in a smaller range of sizes is
occasionally used.
Advantages are: Because its performance has been so thoroughly analyzed
by research and practical application in a wide range of conditions, stone
armor can be designed with an especially high degree of precision and
confidence. The American Society of Civil Engineers' Task Committee on
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